Optical nonmechanical feedback control of ellipticity modulators

ABSTRACT

The invention concerns simplified automatic control of the peak birefringence of a modulator which is operable upon a linearly polarized beam of radiation to derive a modified beam having cyclically varying elliptical polarization. Simplified automatic compensation of the electrical drive signal applied to the modulator drive transducer functions solely in response to variations in the character of a beam of radiation that has passed through the modulator.

Elli U IIlIefl mares Hooper 3,495,912 2/1970 Hooperetal. ..350/1S1X3,450,478 6/1969 Sebestyerl ..356/ll7 FOREIGN PATENTS OR APPLICATIONS[721 Paige Hmpe" Glendm 1,226,328 10/1966 Germany ..356/l17 [73]Assignee: Cary Instruments, Monrovia, Calif.

Primary ExaminerJames W. Lawrence [22] Flled' 1967 Assistant ExaminerT.N. Grigsby [2i] Appl. No.: 630,591 Attorney-White & Haefliger 52 0.5. CI.250/217, 250/225, 350/149, [57] ABSTRACT 356/ l 356/1 17 The inventionconcerns simplified automatic control of the [51] Int. Cl W601 H0139/12, Gozf 1/26 peak birefringence of a modulator which is operableupon a [58] new of Search "350/l50i 151; 356/1 linearly polarized beamof radiation to derive a modified beam 356/116; 250/200 199 havingcyclically varying elliptical polarization. Simplified automaticcompensation of the electrical drive signal applied to [56] ReferencesCited the modulator drive transducer functions solely in response toUNITED STATES PATENTS variations in the character of a beam of radiationthat has passed through the modulator. 3,556,663 1/197] Cary ..356/1 162,064,289 12/1936 Cady ..350/l49 X 4 Claims, 4 Drawing Figures saunas or5 MONOCIIROMHI'IC MODULATOR um 124 770A! LINE RRL Y NEH/V6 NEH/V5POtflR/Zp uenr OPT/C4 A CONVERTER 1 a I 103 ELECTRO- MECHflN/CRL DP/VECONTROL OPTICAL NONMECHANICAL FEEDBACK CONTROL OF ELLIPTICITY MODULATORSThis invention relates generally to instrumentation for use in makingmeasurements with polarized light. More specifically, it concernsimprovements in controlling the magnitude of cyclical birefringence inbirefringencemodulators.

For present purposes, a birefringence modulator is a cyclically drivendevice operable upon a linearly polarized beam of radiation to derive amodified beam or beams having cyclically varying ellipticalpolarization. Such a beam may be utilized for passage through a samplecharacterized as circularly dichroic, whereby differential absorbance bythe sample of left and right circularly polarized light, i.e., circulardichroism, may then be detected and measured. Such a beam also hasutility in other types of measuring instruments, such as those fordetermination of optical rotation produced by specimens. Knownbirefringence modulators for producing such beams include strainbirefringence modulators and electro-optic modulators, among others; forthe sake of simplicity, however, the present invention will only bedescribed as applied to a strain birefringence modulator of advantageousconstruction and mode of operation as disclosed in the copendingapplication of Hooper et al., for U.S. Letters Patent, entitled, StrainBirefringence Modulator and Application," Ser. No. 595,194 now US. Pat.No. 3,495,912. Also for the sake of simplicity, the invention will onlybe described as applied to such a modulator for use in a system designedto measure circular dichroism. However, the control system hereindescribed is equally useful in connection with modulators of othertypes, and in systems for measuring other polarization phenomena.

In order that the ellipticity of polarization of the beam emerging fromthe modulator may assume optimal values for the purposes of a circulardichroism measurement, the cyclically varying birefringence of themodulator must assume a corresponding optimal amplitude. When theellipticity of the emerging beam varies sinusoidally with time, optimumcircular dichroism measurement accuracy is achieved when the peakretardation is about 1 13. For thispurpose, vibration of a cyclicallystrained modulator must be controlled to maintain such a desired peakbirefringence, even though the wavelength of radiation supplied to themodulator is varied. The amplitude of strain variation required in suchmodulator to produce optimum ellipticity of the emerging beam varieswith wavelength. It is a major object of the present invention toprovide for simplified automatic control of the peak birefringence,requiring no wavelength programming. The invention enables simplifiedautomatic compensation of the electrical signal applied to the modulatordrive transducer, solely in response to variations in the character of abeam of radiation that has passed through the modulator.

Basically, the instrumentation comprises first means, operable upon abeam emerging from the modulator with cyclically varying ellipticity, toderive a modified beam whose intensity also varies cyclically, as afunction of the ellipticity of the emergent beam; and second means,responsive only to such intensity variation, for cyclically stressingthe modulator. The relationship between the retardation introduced inthe beam emergent from the modulator and the resulting intensity of themodified beam is made such that the system tends toward a stableequilibrium oscillation condition, at which the peak retardation isoptimal for use in an associated system for the measurement of circulardichroism. More specifically, a component of the modified beam intensitydiminishes as the peak retardation introduced by the modulatorincreases, near a critical value. As will be seen, the system eliminatesneed for any electrical pickup on the modulator to provide feedbackcoupling for driving the modulator, as well as other circuitry, sincethe modulator is driven only in response to variations in the modifiedbeam intensity.

The above-mentioned first means typically includes a plane mirrordisposed to return an elliptically polarized beam from the modulator fora second pass through the modulator, and a fixed-retardance retarder andan analyzer for further processing the reflected beam to produce theabove-mentioned modified beam, as will be explained in detailhereinafter.

The above-mentioned second means typically includes a phototubereceiving the modified beam, and producing an output varying incorrespondence to intensity variations of the modified beam, anamplifier responsive to a component of the output of the phototube, andan electromechanical transducer responsive to the output of theamplifier, mechanically to drive the modulator. No circuitry other thansuch phototube, amplifier, and a transducer, plus appropriate powersupplies and interconnections is needed to establish equilibriumoscillation amplitude of the modulator.

The above improvements are of unusually advantageous use in a systemthat includes a linear polarizer to receive unpolarized monochromaticlight and to produce linearly polarized ordinary and extraordinary beamsincident upon the modulator, the modified beam being derived from one ofthe ordinary or extraordinary beams, the other of the beams passingthrough the modulator and emerging with cyclically varying ellipticalpolarization for passage through a sample characterized as circularlydichroic, together with means to detect and measure differentialabsorbance by the sample of left and right circularly polarized lightcharacteristic of the elliptically polarized beam.

Alternatively, the modified beam may be derived from the same beam usedfor passage through the sample, by means of any conventional beamsplitting device such as, for example, a pellicle mirror.

These and other objects and advantages of the invention, as well as thedetails of illustrative embodiments, will be more fully understood fromthe following detailed description of the drawing, in which:

FIG. 1 is a block diagram of the system of the invention;

FIG. 2 is a perspective showing of one form of modulator usable in theFIG. 1 system;

FIG. 3 is a more detailed showing of a system wherein the invention isof unusually advantageous use, and as applied to the measurement ofcircular dichroism of a test sample; and

FIG. 4 is a graph of modified beam intensity as a function ofdifferential phase delay between the two vector components of the beampassed through the modulator.

FIG. 1 shows a source of approximately monochromatic linearly polarizedlight, whose wavelength may be varied. Operating upon a beam or beams101 of such light is a birefringence modulator means 32a which derives abeam having cyclically varying elliptical polarization. Beam 105 isdirected to utilization means 106, which may for example comprise aspecimen, a photodetector, and associated electronic circuitry for themeasurement of circular dichroism. Modulator means 32a also derives frombeam 101 another beam 105a, also having cyclically varying ellipticalpolarization. As will be explained in detail hereinafter, in a preferredembodiment the polarization condition of beam 105a is not the same asthat of 105, the difference resulting from the fact that modulator 32aoperates only once upon beam 101 to produce beam 105, but operates twicein sequence upon beam 101 to produce beam 105a.

Beam 105a is directed to optical converter 102 which derives a modifiedbeam 103 whose intensity varies cyclically as a function of theellipticity of beam 105a, and thus of beam 105. Beam 103 is directed toelectromechanical drive control means 104, responsive only to theintensity and time phase of beam 103, for controlling modulator 32a. Inthe case discussed here, modulator means 32a is of the strainbirefringence type, and drive control means 104 produces an electricalcontrol signal in feedback path 104a, for controlling cyclical stressingof the modulator, to produce cyclical strain and thus cyclicalbirefringence therein. Optical converter 102 is such that a component ofthe modified beam intensity 103, preferably that component whosefrequency is the modulator operating frequency, diminishes as theamplitude of oscillation of the differential retardation of beams 105and 105a increases, near a specified level. As a consequence the loopcomprising modulator means 32a, converter 102 and control 104 tendstoward oscillation with a characteristic equilibrium amplitude.

Referring next to FIG. 2, the modulator illustrated may be used in theFIG. 1 system, and comprises a body, as for example plate 10, adapted topass electromagnetic radiation directed thereto along a predeterminedpath, say path 11 along the Z axis. The body or plate is characterizedas becoming optically plane birefringent, under the influence of appliedstress. Thus, light which enters the material of the body linearlypolarized emerges therefrom elliptically polarized. FIG; 2 shows theelectric vector E of linearly polarized light incident upon the plate,the vector having component B, lying in the direction of the strain axisX of the plate 10, and component E extending normal to component B,Also, E extends at a 45 angle to each of the axes X and Y. Under theseconditions the phase difference between the E, and E, component wavesemergent from the plate can be made to vary cyclically by generatingwithin the plate a cyclically varying birefringence, so that the degreeof elliptical polarization of emergent light will vary between plus andminus values. The 45 orientation constitutes a special case in which theplane-birefringent plate only produces ellipticity and does notintroduce rotation. The transparent plate, which may typically consistof fused silica or Suprasil," is made appropriately cyclicallybirefringent by cyclically stressing the plate along its X or strainaxis, indicated in FIG. 2.

FIG. 2 shows drive structure connected in mechanical oscillationtransmitting relation to the plate to effect plate vibration along the Xor strain axis, as described in the above-mentioned Hooper et al.application. The transducer form of drive structure in FIG. 2 includessuitable piezoelectric elements 13 attached to the plate's oppositeedges a at center nodal points, i.e., half way along the X-axisdimension of the plate. Merely by way of example, the drivers mayconsist of barium titanate, being about 0.125 inch thick, 0.200 inchwide i.e., the same as the plate thickness) and about 0.250 inch long.They may be cut from commercial grade piezoelectric material known asPZT-4, a product of the Clevite Corporation.

The drivers have opposed conductive coatings 13a and 13b to whichelectrical connections are made at Ma and 14b to transmit the actuatingsignal to the devices, thereby to cause the plate to vibrate resonantlyalong the X-axis or plate length (strain) dimension. With a sinusoidaldrive signal input, the sinusoidal strain which the plate undergoesmakes the plate become plane birefringent, with the magnitude and signof the birefringence varying sinusoidally in time. If the platedimensions are fairly large in relation to the optical beam crosssection dimensions, the strain and hence birefringence are quitehomogeneous over the small cross section lla intersected by the beampath 11.

As will appear, electrical oscillations are transmitted at Ma fromamplifier-oscillator 21 to the drivers so as to maintain the peakretardation (in wavelengths) introduced by the modulator platesubstantially constant, at a predetermined value related to otheroperating parameters, as explained more fully hereinafter, the platepreferably vibrating longitudinally at its fundamental resonantfrequency. Accordingly, the peak retardation at cross section 11aremains constant.

FIG. 3 illustrates the use of the FIG. 2 plate in a system for measuringcircular dichroism of a sample 25. The element 26 designated lightsource emits electromagnetic radiation as a continuum over a relativelybroad range of wavelengths, which may be in the visible, infrared and/orultraviolet portions of the electromagnetic spectrum. The term light"will be used to designate any of such radiation. The monochromator 27has the function of selecting from this continuum a narrow band ofwavelengths for use in measuring the circular dichroism of the sample,as is known. Depending upon the application, the monochromator may be arelatively coarse apparatus, or a fine high-resolution device such asthat employed in Model 60 Automatic Recording Spectropolarimeterproduced by CARY Instruments, Monrovia, California. A scan drive 28 maybe coupled at 28a to the monochromator to cause it sequentially toselect different narrow wavelength bands of light for transmission at29, the arrangement being such that the nominal or central wavelengthsof the selected bands form the locus of a smoothly varying function oftimea monotonic functionof approximately constant slope.

From the beam 29 leaving the monochromator, a substantially linearlypolarized component is selected by the polarizer element 30 andtransmitted at 31, as the ordinary beam. See in this regard the Model 60apparatus above identified, as well as the article Circular DichroismTheory and Instrumentation," [by Abu-Shumays and Duffield, AnalyticalChemistry, Vol. 38, June l966. The extraordinary beam 60 is utilized ina manner which will be described.

Linearly polarized light at 31 is incident upon the vibrating :modulator32, of the construction seen in FIG. 2, so that light leaving themodulator at 33 is in general elliptically polarized, i.e., havingelectric and magnetic vectors each of whose tips describes an ellipse,in time, when projected into a plane perpendicular to the direction Z oflight propagation. Such light may be considered equivalent to twocounterrotating circularly polarized components vectorially added, therelative magnitudes of the two components determining the magnitude ofthe ellipticity. The algebraic sign of the ellipticity is determined bythe direction of rotation of the resultant vector, i.e., by the sense"of the larger circularly polarized component.

Light leaving the modulator at 33 is incident upon the sample 25, whichabsorbs unequally the circularly polarized components of oppositesense," so that, as the ellipticity periodically changes sign, the totalamount of light incident on the phototube undergoes a correspondingperiodic variation, i.e., larger when the light passing through thesample possess a net circularly polarized component of the senseabsorbed to lesser degree by the sample, and smaller when the netcircularly polarized component is of the sense absorbed to greaterdegree by the sample.

A phototube 34 receives both fluctuating and steady (or constant)components of light flux transmitted from the sample at 35 so that thecurrent output of the tube contains both fluctuating and DC components.The fluctuating components are substantially sinusoidal AC, onecomponent of frequency equal to the fundamental frequency of themodulator plate, and other components having frequencies which are oddmultiples of the fundamental, and corresponding in magnitude to thedifference between the transmission levels for the circularly polarizedcomponents of opposite sense. There may also be small relativelyinsignificant AC current components due to parasitic vibration of theplate 10, at frequencies other than the plate fundamental frequency. TheDC component corresponds in magnitude to the average or meantransmission of the sample for light in general.

The phototube output at 36 is fed to readout electronic circuitry 37,which may be comparable to that described in US. Pat. No. 3,257,894 toGrosjean, with the exception that the carrier frequency is thefundamental vibratory frequency of the modulator plate 10. Asynchronizing input signal to circuitry 37 is shown as derived at 38from the amplifier 21 whose output at 23 controls the piezoelectricdrivers 13, for use in a detector embodied in block 37 to derive adetected version I,, of the AC output component from tube 34. Thereadout circuitry also derives the ratio of I, to I the latter being aversion of the DC output component from tube 34. The value of the ratiois very nearly proportional to the actual value of circular dichroism ofthe sample.

To expedite the electronic determination of the ratio I, to I, in thebest embodiment now known for determination of circular dichroism ofabsorbing samples, or over a wide range of wavelengths, it is desirableto provide an automatic gain-control feature which maintainsapproximately constant the DC component of the phototube output, at apoint in the system ahead of that at which the ratio determination isperformed.

Such a gain-control feature maintains the signal levels in the ratioingdevice within suitable operating limits. In one such suitable system theautomatic gain-control is in the form of an automatic regulator for thedynode voltage, and maintains the DC current I, constant to within Ipercent of a nominal value. (In such a system, the absolute value of theAC component I is itself a measure of the circular dichroism, within theaccuracy of the regulating system, i.e., 1 percent.)

An output signal at 39 from the readout 37, and proportional to l ll isfed to the actuator 40 controlling the position of an ink pen 41a of astrip chart recorder 41, thereby to record a value which is an excellentapproximation to the circular dichroism of the sample. The scan drivemotor 28 referred to above also drives a platen 42 translating the chartpaper 43 in a direction 44 normal to the motion of the recording pen, sothat the position of the pen longitudinally along the chart paper is acontinuous known function of wavelength. Thus an ink tracing of circulardichroism versus wavelength is produced.

As mentioned in the introduction, the invention has as one importantpurpose the provision of a simplified automatic control of peakretardation at the modulator, thereby eliminating need for a pickup atthe modulator or for independent and objectionably inaccurateprogramming to compensate the modulator for changes in wavelength of themonochromatic light transmitted at 29 and 31. In this regard, use ismade of the extraordinary ray 60 which like the ordinary ray 31 islinearly polarized, and corresponds to part of beam 101 in FIG. 1. Theusage is such that a modified beam 103a (corresponding to beam 103 ofFIG. 1) is derived and characterized in that its intensity variescyclically as a function of the cyclical strain amplitude in plate 10.

The first means as represented by block 102 in FIG. 1 may for exampleinclude a device such as a reflector 62 positioned to receive the beam60 after it has passed through the vibrating plate at 61, and to returnthe beam at 63 back through the plate, thereby doubling the phaseretardation. By disposing the mirror 62 for substantially normalincidence of the beam 61, the retardation effects introduced onreflection at the mirror surface may be maintained at a negligiblelevel. The emergent beam 63a is then passed through a fixed retarder 64,as for example a fixed nominal quarter-wave retarder characterized inthat incident linearly polarized light (properly oriented) emerges fromthe retarder circularly polarized. Retarder 64 advantageously has itsfast and slow axes oriented one parallel and one perpendicular to thestrain axis X of plate 10. The emerging beam 65 then passes through ananalyzer 66 which transmits only the component of incident lightpolarized parallel to the transmission axis of the analyzer. Themodified beam 103a emerges from the analyzer, for intensity detection bymeans including a photomultiplier 67.

In order better to understand the relationship between the resultantintensity of beam 103a and the operation of the plate 10 to cyclicallymodify the differential phase retardation between components of theelectric vector characterizing the linearly polarized extraordinary beam60, consider the following three cases, and refer to the FIG 4 graph.Assume now the extraordinary beam 60 to be polarized perpendicular tovector E in FIG. 2, at an azimuth of 45 from the strain axis X of theplate 10, so that beam 60 may be resolved into two equal vectorcomponents, one parallel and one perpendicular to the induced strainaxis.

While a great variety of relative orientations of elements 64 and 66, inconjunction with the polarities of electrical circuitry adapted toreceive signals from phototube 67, will produce equivalent results, forthe purposed of the analysis to follow hereunder certain simplifyingassumptions are made: (a) The retardation introduced by the modulator ispositive when the fast axis of the modulator is parallel to the fastaxis of the fixed retarder. (b) The extraordinary beam 60 is polarizedperpendicular to the transmission axis of the analyzer 66.) Case I Whenthe modulator plate is undergoing zero strain, there is no differentialphase delay between the two vector components of the beam. Beam 60passes through the modulator at 61 and 63 still linearly polarized, isincident on retarder 64 polarized at an azimuth of 45 with the fixedfast axis of the retarder, and exits from the retarder 64 at 65 as acircularly polarized beam, containing one-half of the total flux (theother one-half of the flux was lost to the ordinary ray 31). In passingthrough the analyzer, the beam becomes plane polarized, halving the fluxcontent or intensity of beam 65, so that only one-quarter of the totalflux arrives at the photomultiplier 67. Analyzer 66 may be of the Rochonor Senarmont type; in such a system, for the ideal case underconsideration, half of the energy at 65 impinging upon analyzer 66 islost as an extraordinary beam to the mask or light trap 81. Thisestablishes point A on the FIG. 4 graph.

Case 11 When the modulator is undergoing strain to instantaneouslyproduce a 45 phase displacement between the two vector components of thelinearly polarized extraordinary ray 60, these two components after twopasses through the modulator have thus undergone relative phaseretardation, and the beam is circularly polarized. The fixed retarder 64converts this beam to a linearly polarized beam at 65, having its vectorin alignment with the transmission axis of the analyzer 66. As a result,there is no loss of flux in the analyzer, and the intensity of lightfalling on the photomultiplier is 0.5, represented at point B on thegraph.

Case 11] When the modulator is undergoing strain to instantaneouslyproduce 90 phase displacement between the two vector components in beam60, after two passes through the modulator 180 of phase delay have beenproduced and the beam falling on the retarder 64 is plane polarized withits vector rotated 90 relative to its position before passage of thebeam through the modulator. The beam at 65 is circularly polarized, andof opposite sense relative to that of case I, so that as in case I,one-half of the flux is eliminated in the analyzer, leaving beam 103awith only one-fourth of initial flux content. This establishes point Cin the graph.

Complete analysis shows that flux intensity I of beam 103a is acontinuous sinusoidal function of twice the differential phase delay 20applying to negative differential phase delay as well as positive. Thisfunction is:

FIG. 4 also suggests that as phase delay is cyclically generated, theintensity of light falling on the photomultiplier changes cyclically.Analysis of the intensity waveform shows that the intensity variationscontain the fundamental frequency of the phase delay fluctuations of themodulator, multiplied by the Bessel function J (20 so that if theamplitude 0 of the fluctuating phase delay 0,, reaches about thefundamental component of the light intensity fluctuation is reduced tozero. With further increase beyond 110 of peak optical phase delayfluctuation, the fundamental in intensity fluctuation reappears, withreversed sign (in other words, change of time phase), providing a nullin the fundamental component of intensity at about 1 10 peak phasedelay.

The reverse in sign of the fundamental component of intensityfluctuation, in passing through the null, results in negative feedbackaround the loop comprising phototube 67, amplifier 21, drivers 13b,plate 10, retarder 64, and analyzer 66. This change in sign or timephase of the fundamental component of intensity fluctuation of the lightbeam 103a is manifested in a corresponding change in sign, or phase, ofthe electrical signals at 70 and 23. The result is a stable equilibriumvibration amplitude of plate 10, such as to produce peak relativeoptical phase delay of approximately 1 10.

While the retarder 64 has been chosen as a quarter-wave device becausethis retardation maximizes the fundamental component of the signal atthe phototube, and this maximizes the sensitivity of the automaticcontrol mechanism, retarder 64 can also be chosen to have any otherretardation whose sine is not zero, and the device will operate, but notwith maximum sensitivity.

Retarder 64 need not be a device whose retardance as measured inwavelengths is constant for light of different wavelengths. In view ofthe preceding statements regarding maximization of sensitivity,variation with wavelength of the retardance of the fixed retarderresults only in a variation with wavelength of the sensitivity of theautomatic control mechanism. The retarder 64 may be chosen to be aquarterwave device at some wavelength roughly central to the wavelengthrange over which the instrument is to operate.

Analyzer 66 has been described as of the Rochon or Senarmont type;alternatively, a pile-of-plates or Polaroid sheet analyzer may be used.Such a substitution would result in modification of the three-caseanalysis presented above, as regards the magnitude of the light fluxreaching phototubes 67, in a manner understandable to those skilled inthe art.

Referring back to FIG. 3, the output 70 of the photomultiplier 67 drivesthe oscillator amplifier 21. Upon starting of the system, theoscillations of plate 10 increase in magnitude. The light flux in beam103a results in photomultiplier output at 70 controlling the oscillatoramplifier 21 driving the plate 10. When the optical phase delay of themodulator approaches the critical value of 110", the fundamentalcomponent of the light intensity fluctuation is diminished andapproaches zero, whereby the output of the oscillator amplifier 22diminishes. This in turn acts to restore the fundamental component andto control the amplifier 21 in such manner that the system reaches anequilibrium condition proximate an optical phase delay of about 1 10.

It is found that for maintenance of optimum elliptical polarizations ofbeam 33, for the particular circular dichroism measurement systemdescribed in the above-referenced copending application of Hooper etal., the amplitude of phase delay introduced by the plate 10 should beapproximately l 13. Thus, the system operates in a steady state underconditions favoring highly precise measurement of circular dichroism insamples. Similar arrangements of fixed retarders and analyzers may beemployed to produce similar steady-state cyclical variation ofbirefringence in modulators, but with other amplitudes closelyapproximating ideal modulation amplitudes for other types of measurementsystems, as will be apparent to those skilled in the art.

I claim: 1. [n apparatus for modulating the ellipticity of polarizationof a beam of light, including a modulator operable upon an incidentlinearly polarized beam of radiation to produce at least one emergentbeam having ellipticity cyclically varying at a frequency, said beamcharacterized by sequentially different wavelengths of radiation, andincluding first means, operable upon one such emergent beam, forderiving a modified beam whose intensity varies cyclically as a functionof such cyclically varying ellipticity the improvement comprising:

second means, responsive to a component of said cyclically varyingintensity, for controlling said modulator to maintain the amplitude ofthe cyclical variation of said ellipticity at a predetermined level ateach of said sequentially different wavelengths; said component beingcharacterized by oscillation at said frequency, the amplitude of saidoscillation approaching a null at said predetermined level, and

said second means including a photodetector responsive only to saidmodified beam, the photodetector having an output, an amplifier havingan input directly connected to the photodetector output, the amplifierhaving an output, and a transducer having an input directly connected tothe amplifier output to drive the modulator.

2. The improvement of claim 1 wherein:

said first means comprises a fixed retarder and a polarizing elementreceiving said one emergent beam. 3. The control system of claim 2including Instrumentation for the measurement of circular dichroismconnected with said apparatus to receive polarized light emerging fromsaid retarder.

4. The improvement of claim 2 wherein the frequency of the photodetectoroutput is the same as the frequency of the amplifier output applied tothe modulator.

1. In apparatus for modulating the ellipticity of polarization of a beamof light, including a modulator operable upon an incident linearlypolarized beam of radiation to produce at least one emergent beam havingellipticity cyclically varying at a frequency, said beam characterizedby sequentially different wavelengths of radiation, and including firstmeans, operable upon one such emergent beam, for deriving a modifiedbeam whose intensity varies cyclically as a function of such cyclicallyvarying ellipticity the improvement comprising: second means, responsiveto a component of said cyclically varying intensity, for controllingsaid modulator to maintain the amplitude of the cyclical variation ofsaid ellipticity at a predetermined level at each of said sequentiallydifferent wavelengths; said component being characterized by oscillationat said frequency, the amplitude of said oscillation approaching a nullat said predetermined level, and said second means including aphotodetector responsive only to said modified beam, the photodetectorhaving an output, an amplifier having an input directly connected to thephotodetector output, the amplifier having an output, and a transducerhaving an input directly connected to the amplifier output to drive themodulator.
 2. The improvement of claim 1 wherein: said first meanscomprises a fixed retarder and a polarizing element receiving said oneemergent beam.
 3. The control system of claim 2 includinginstrumentation for the measurement of circular dichroism connected withsaid apparatus to receive polarized light emerging from said retarder.4. The improvement of claim 2 wherein the frequency of the photodetectoroutput is the same as the frequency of the amplifier output applied tothe modulator.